The present invention relates in general to tunable electrical filters of the type comprising an RC (resistance-capacitance) network for determining the effective filter time constant, and in particular to tunable filters which include at least one operational amplifier in said network.
In the simplest of filters of the above type, the network may consist of a single resistor and a single capacitor. In general, however, much more complex networks are required to meet the requirements of many filter applications calling for exacting frequency response characteristics. For example, this is especially true of low-pass filters for use ahead of a read out device, such as a chart recorder or digital display in scanning spectrophotometers.
The fundamental parameter that plays a part in any filter characteristic is of course the effective time constant. In the simplest of filters, it is merely the product of one resistor value and one capacitor value. In more complex filters, the RC network may include several RC combinations, each with its own time constant, so that the effective time constant is the result of the interaction between the various combinations.
Tunable filters, in which the effective time constant may be varied over a predetermined range, are often required for the purpose of matching changing operational requirements. It is easy to imagine the complexity and attendant functional unreliability of the switching system required to select the large number of RC permutations needed to cover even a modest range in, say, a third-order filter. Not surprisingly, therefore, variable time-constant filters have hitherto tended to be of the simplest first-order kind, providing a generally unsatisfactory performance in all but the less demanding situations.
The feature in the low-pass characteristic of a tunable noise filter for use in a scanning spectrophotometer, therefore, which is of particular interest is the cut-off frequency, which sets the upper limit of the noise bandwidth and may be altered by inversely varying the effective time constant of the filter. It may be proved mathematically, although it is easy to appreciate it intuitively, that optimum filtering, i.e. maximum noise attenuation with minimum signal distortion, may be obtained if the filter is arranged to have a comparatively sharp cut-off around the highest frequency expected in the signal. Now, for any sample under analysis the highest signal frequency is naturally determined by the rate at which the ordinate information is made available, or in other words by the rate of abscissa generation, i.e. the rate of wavenumber or wavelength scan. The faster the scan, the higher must be the filter cut-off frequency to cope with it; but the higher the cut-off frequency and, consequently, the wider the bandwidth, the greater the noise.
In an application of particular interest, a scanning spectrophotometer, resolution, signal-to-noise and scan speed are three major interrelated parameters, resolution being most frequently regarded as the leading parameter. In practice, the operator typically decides first on the minimum resolution required for the analysis in hand and adjusts the filter bandwidth to ensure that the signal-to-noise ratio is adequate. The scan speed required then becomes a consequential adjustment in that it must not exceed the limit at which the incoming information rate would be too high for the cut-off frequency selected. This procedure ensures that a satisfactory spectrum is obtained in a minimum of time.
Resolution is of course improved by reducing the opening of the monochromator slits, but this reduces the amount of energy reaching the photometric detector and leads, therefore, to a lower signal-to-noise ratio of the detector output signal, which is compensated for by decreasing the filter bandwidth, i.e. by increasing filtering. Because of the greatly differing spectra handled by a spectrophotometer, a large range of resolution values must be provided for in its design. This inevitably means that a wide selection of filter bandwidths must also be included, which is tantamount to saying that the range of filter cut-off frequencies and consequently effective filter time constant must be large, typically in the region of 100:1.
In prior art scanning spectrophotometers an attempt has been made to produce variable bandwidth filters in which different value resistors and capacitors are switched in and out through an operator control. Because of the intolerable complexity that would result with higher order filters, the scheme has been confined to simple first order RC filters, which unfortunately do not allow the optimum filtering conditions desired to be closely approached.
It is a primary object of the present invention to provide a tunable filter in which all the time-constant determining capacitors of an RC network forming part of the filter are adapted to be switched at a variable ON-time to OFF-time ratio, the time constant of the network increasing as the ratio decreases.
It is a further object of the present invention to provide a spectrophotometer incorporating the said tunable filter in the form of a low-pass noise filter.